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Use darks, bias and adaptive dark subtraction. CCDstack may do it. Images Plus does it. Maxim probably does as well.
Take a shot of the same exposure details (length, ISO etc) with the lens cap on at night so no light leaks. Ideally take 16 but less would work.
Sigma reject combine these darks to make a master. Subtract a bias master from them during the procedure.
A bias master is same settings except the shortest exposure time. Its a picture of the noise from the electronic readout of the camera.
Take several, sigma reject combine.
Ideally you do the above at the same temperature you image at. Noise varies with temperature. Adaptive darks help overcome that but try to minimise the diffference.
Once you've made your dark master try to standardise your exposure and ISO settings so they are not all different to make the dark work best.
Work out what that is for your camera. A 60Da is probably around ISO1200 max and depending on your light pollution/ tracking accuracy of your mount maybe 5 or 10 minutes. Bad light pollution needs shorter exposure lengths and a light pollution filter.
Also, why can't you dither with them both going? There's only one guide scope and camera. Can't you dither that and it dithers both?
I hope this helps.
Greg.
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